PROBLEM TO BE SOLVED: To provide a novel method for measuring the thermo-optical constant of a polymeric material or a polymeric thin film used for a polymeric light wave guide path device, or the like.
SOLUTION: A laser beam emitted from a light source 1 is transmitted by a λ/2 plate 2 to obtain TE-mode light or TM-mode light. The light is evenly polarized by a polarizer 3, then converged by a lens 4, and made incident at an angle θ on the lattice-shaped polymeric thin film 12 constituting a polymeric light-wave guide path element 10 on a rotary stage 5. The rotary stage 5 is rotated to change the angle θ, thereby providing reflected light with a plurality of modes. Reflection angles θ0 and θ1 of the reflected light of the respective modes are measured to obtain refraction factors ng. The above operation is conducted at a plurality of different temperatures to obtain refraction factors ng corresponding to the respective temperatures T. The thermo-optical constant dng/dT is induced from the slope of a graph expressing a relation between these.